The present invention relates to the detection of bubbles or air in transparent tubing and, in particular, a sensor for detecting both air-liquid and liquid-air transition.
Bubbles or air pockets in transparent tubing result when liquid passing through the tubing runs out or a break in the tubing causes the introduction of air into the tubing system. Many modern laboratory operations require liquids to be pumped or passed through tubing connecting a solvent reservoir to an instrument or other operation. Pumping air through many instruments or introducing air in certain reactions can cause damage to instrumentation or in some cases create a significant safety hazard. In these situations, it is very important that tubing transporting the solvent be monitored for the presence of air just before the solvent enters an instrument or is added to the operation, such as a chemical reaction, so that action can be taken to prevent the introduction of the air.
A typical setup to monitor for air in solvent lines involves placing an energy source on one side of the tubing carrying the solvent and a receiver sensitive to the energy emitted by the energy source on the other side. For example, see U.S. Pat. Nos. 4,314,484 and 4,487,601, the entire disclosures of which are incorporated herein by reference. One common energy source that can be used as an example is a light emitting diode and its appropriate energy receiver is a photo transistor. Such a bubble detector works on the theory that when liquid is present in the tubing the photo detector produces a set output signal, the level of which can be measured to establish a "liquid present" output level. A reference voltage (or current) is then set which is slightly offset from the "liquid present" voltage and the two voltages are continuously compared. If a bubble develops in the tubing and passes between the light emitting diode and the photo detector, the output voltage of the photo detector changes causing it to fall below (or above) the reference voltage, thus signaling the presence of air.
This prior art detection approach has three principle disadvantages. First, it requires that the reference voltage be adjusted, usually manually, every time any part of the system that affects the output of the photo detector is changed. The output of the photo detector is affected by many properties such as the clarity of the liquid and the material of which the tubing is made including its transmittivity, wall thickness, and diameter. Also, other factors such as alignment of the transmitter/receiver pair and their relative efficiencies affect the output voltage of the photo detector. This heavy dependence on the system configuration requires that the reference voltage of the bubble detector be adjusted for each use.
A second disadvantage is that the prior art detection approach does not compensate for slow drift of the "liquid present" signal level. For many operations the liquid monitored in the tubing changes over the course of time, such as with gradient column chromatography where the starting solvent can be water but changes to acetonitrile over a certain time period, typically 30 minutes. Changing the solvent monitored in the tubing causes a change in the level of the "liquid present" voltage. If the change is significant enough the "liquid present" voltage can cross the reference voltage causing a false indication that air is present.
The third disadvantage of the prior art detection approach is that the difference between the output voltage from the photo detector when liquid is present and when air is present can be very small. This situation requires that the reference voltage be set very close to the "liquid present" voltage, close enough that even normal amounts of electrical noise can cause false detection of air.
An additional disadvantage is present in the design of some bubble detection devices, namely that they only detect the transition from liquid to air and not the transition from air to liquid. This is important for applications that do not turn off equipment when air is detected, but rather open a valve to purge air from the system. These applications require information not only that air is present to initiate the purge operation, but also that the transition from air to liquid has occurred to indicate the purge is complete.